1. Field of the Invention
The present invention relates in general to liquid crystal image display apparatus and, particularly, to a liquid crystal image display for displaying a still black and white image having intermediate gray tones.
2. Description of Prior Art
It has been well known for some time to use liquid crystal displays (LCD) for alpha-numeric indicators in watches, calculators, and the like. In such applications, the liquid crystal display need only provide a black and white representation of the data being displayed. More recently, liquid crystals display have been arranged in much larger arrays and used to produce still pictorial images that are derived in some cases, from a video signal. In order to produce realistic pictorial images, various intermediate gray tones between the black and white are required, these gray tones make up what is commonly known as a gray scale. The typical approach to producing an appropriate gray scale for realistic images is to pulse width modulate the video signal used to drive the liquid crystal display, thereby obtaining the desired intermediate gray tones between black and white.
The presently known approach to achieving the desired gray tones by pulse width modulating a video signal uses an analog-to-digital converter to derive digital gray scale information, and a number of shift registers, in combination with respective multiplexers, are used in both horizontal and vertical directions on the LCD array. The derived digital gray tone signals are applied to the liquid crystal display array in the vertical direction using a shift register and multiplexer and, in the horizontal direction, complicated switching circuitry is required between the shift registers containing the digital values and a multiplexer used to apply the shift register contents to the liquid crystal display array. In regard to these multiplexers, typically one multiplexer has as many stages as the number of picture elements in one horizontal period and, similarly, the other multiplexer has as many stages as the number of scan lines in the vertical direction.
The presently known system prevents a decrease in quality of the crystal due to electrolysis by driving the liquid crystal display with what is essentially an alternating current signal. In the known system, the tone of the LCD is controlled by the effective value of the applied voltage, however, because the driving signals are pulse width modulated the effective frequency of the signals will also vary according to the tone to be reproduced. This adversely affects the so-called operational margin of the liquid crystal, which is another way of referring to the sensitivity or the degree to which a liquid crystal responds to changes in input signal level. Changes in the operational margin of the liquid crystal will prevent a correct tone display from being achieved.
Additionally, the circuit elements and circuit construction required for the analog-to-digital converter used in the above-described system is quite complicated, also, a shift register is required for each bit of the output signal of the analog-to-digital converter. Similarly, in producing the pulse width modulation signal, a plurality of individual switch circuits are required that is equal to the number of picture elements, thereby further increasing the complexity of the circuit.